By Becky Morton
Business reporter, BBC News
The UK's competition watchdog is investigating Asos, Boohoo and Asda over claims about the sustainability of their fashion products.
If the firms are found to have made misleading claims, the Competition and Markets Authority (CMA) said it would not hesitate to take action.
Concerns include the use of vague language which may suggest collections are greener than they actually are.
The companies said they were committed to providing accurate information.
The CMA's move is part of an ongoing investigation into potential "greenwashing", when firms brand something as sustainable when this is not the case, and follows concerns about the way the companies' products are being marketed as eco-friendly.
An initial review in January identified concerns about potentially misleading claims, including companies making broad statements about the use of recycled materials in new clothing with little or no information about the basis for those claims.
Other concerns which will be investigated include whether:
Sarah Cardell, interim chief executive of the CMA, said: "People who want to 'buy green' should be able to do so confident that they aren't being misled. Eco-friendly and sustainable products can play a role in tackling climate change, but only if they are genuine."
"Should we find these companies are using misleading eco claims, we won't hesitate to take enforcement action – through the courts if necessary," she added.
"This is just the start of our work in this sector and all fashion companies should take note: look at your own practices and make sure they are in line with the law."
Possible actions which could be taken by the CMA include securing undertakings from companies to change the way they operate or taking firms to court.
The move comes after the CMA published its Green Claims Code in September 2021, which aims to help businesses understand how to communicate their green credentials, while avoiding the risk of misleading shoppers.
Dr Elaine Ritch, a lecturer in marketing, fashion and sustainability at Glasgow Caledonian University, said greenwashing was "prevalent" in the fashion industry and used to encourage more consumption.
She gave the example of some companies branding fabrics sustainable because they were organic, but in fact they were very cheap so would wear out quickly.
However, she told the BBC she did not think the CMA investigation would have a significant impact and called for tougher regulations, including fines for companies who made misleading claims about sustainability.
Asos said it was "committed to playing its part in making fashion more sustainable" and was cooperating with the investigation.
Rival online fast fashion brand Boohoo said it would "continue to work collaboratively with the CMA".
The company said it recognised the need to provide customers with "straightforward and no-nonsense information".
Asda said: "We know how important it is that our customers can trust the claims we make about our products, which is why we ensure the statements we make can be supported by industry accreditations."
The supermarket chain added that it was "ready and willing to answer any questions the CMA have".
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By Becky Morton